The present invention relates to the manufacture of hypochlorous acid and, in particular, a process and apparatus for the on-site manufacture of hypochlorous acid in a salt-free solution.
Hypochlorous acid has long been contemplated as a multipurpose processing agent and chemical intermediate. Because of its powerful and selective oxidation properties, hypochlorous acid has been proposed for the bleaching of pulp and textiles, particularly as described in U.S. Pat. No. 2,178,696 entitled "Material Treatment" and issued on Nov. 7, 1939. It has also been proposed for the manufacture of hypochlorites, chlorohydrins and chloroisocyanurates, also because it is such a powerful oxidant, particularly as described in U.S. Pat. No. 1,510,790 entitled "Manufacture of Chlorohydrins" and issued on Oct. 7, 1924.
Hypochlorous acid has also been thought to be the active bacteria and microorganism killing species in water treatment through application of chlorine, hypochlorites or chloroisocyanurates, particularly as described in "The Handbook of Chlorination" by G. E. White in the 1972 edition.
Notwithstanding the recognition of hypochlorous acid as a desireable chemical agent and intermediate, the commercial use thereof has been limited because of its inherent properties and the difficulty in manufacturing a suitably pure product in a competitive economical process.
Hypochlorous acid is a relatively unstable compound. In even a dilute form, the solution decomposes with increasing rapidity as the temperature increases. It also decomposes upon exposure to light. This requires that shipment of commercial quantities from off-site locations take place under tightly controlled temperature, container and timetable restrictions. The shipping also must be substantially in accordace with demand to limit the change in chemical quality from the time of manufacture to the time of use. The problems of concentration are particularly troublesome inasmuch as the product can be explosive in concentrated solution. These problems are further accentuated if the solution contains impurites such as chlorides or chlorates.
Thus the preference for the commercial use of hypochlorous acid has been for on-site manufacture in accordance with the compound demand. These efforts have been typified by complex and capital intensive processes and apparatus. They have also been troubled by the complexities in obtaining the required product purity.
It has also been proposed that the hypochlorous acid be directly prepared by reaction of chlorine and water or reaction of a chlorine solution with a strong base such as caustic or lime. While both of these approaches produce the solution, they also produce undesireable chlorides which increase the decomposition rate, complicate the subsequent manufacturing processes and require the aforementioned prompt usage and storage conditions.
It has also been proposed to produce the hypochlorous acid solution through the intermediate chlorine monoxide, as disclosed in U.S. Pat. No. 2,157,524 and 2,157,525, both issued on May 9, 1939, and thereafter dissolving the chlorine monoxide in water to form the acid solution. Such a solution, however, contains the undesireable chlorides.
Additionally, it has been proposed to react the chlorine monoxide with sodium carbonate to form hypochlorous acid vapor which is then dissolved into solution as described in U.S. Pat. No. 2,240,344 issued on Apr. 29, 1941. In commercial use the process has been modified to use steam as a supplemental water source and to use anhydrous sodium carbonate as a reactant, still using the elaborate towers or tumbling reactors. However, both approaches yield only a low conversion to chlorine monoxide with a substantial amount of unreacted chlorine, thus being limited to those facilities having a sufficiently large on-site demand to justify the investment.
A large unsatisfied demand thus exists for a simple, high yield process and apparatus adaptable for varying production needs which can produce on-site a chloride-free hypochlorous acid solution.